Transverse wakefields in the linac of the SLAC Linear Collider (SLC) have been observed to enlarge the effective emittance of beams which are not properly centered in the accelerating structure.‘12 A fast feedback system has been constructed to minimize the enlargement under changing conditions by controlling the beam launching parameters. Theoretical aspects of this transverse feedback system are reviewed as well as the design of the beam sensors, launch controllers, communication equipment and data processing micro-computer. A variety of beam observations have been made. They show that dispersion as well as wakefield effects are important. In the near future the fast transverse feedback system will be beam tested, and algorithms tailored to the noise environment of the SLC will be tried. System Overview The transverse feedback system of the linac has components for both slow and fast beam control. Trajectory correction over the entire linac2T3 involving 550 dipoles and 275 beam position monitors (BPM) is performed about every 30 minutes. In this way, trajectory errors are kept below 200 pm rms. Linac input and output launch parameters (z, z’, y, y’) are corrected3 about once per minute in order to maintain injection from the Ring-ToLinac transport line (RTL) into the linac and from the linac into the ARC system. Both of these controls rely on information from local beam position monitors with resolutions of order 50 pm and ignore current-dependent phenomena. At high beam intensities transverse wakefields make the beams very sensitive to small changes in launching position and angle. As a result, beam positions must be controlled to a few microns’ which is beyond the sensitivities of the local beam monitors. Furthermore, the parameters are expected to change sufficiently rapidly that the VAX control system cannot simultaneously handle the feedback load above all the other necessary control functions. Thus, a stand-alone micro-computer is needed as well as an independent feedback process. The fast transverse feedback system has sensors at the end of the linac where the transverse beam distortions are most pronounced. The control elements are located in the e+ and eRTL’s because the beams are most sensitive to errors (and to corrections) in the early part of the linac. As an illustration, in the absence of transverse wake fields a beam with a bunch length of 1 mm and 5 x 1O’O particles which is injected into the linac off axis by 10 pm at p = 10 m will oscillate with an amplitude that damps via acceleration to about 2 pm at 50 GeV. IIowever, with transverse wakefields a particle behind the bunch center by a,/2 will be driven to an amplitude of about 120 pm by the end of the linac. A particle 2a, behind the bunch center will have an amplitude over 1 mm. Integrating over the bunch, a 10 pm injection error will be manifested as a 100 pm centroid shift at the end of the linac plus tails which extend to 1 mm. The